Leakage detection systems are used to detect undesirable spills of contaminating substances such as oil and gasoline from storage containers and tanks so that countermeasures can be taken.
EPO 0 337 630 A1 relates to a leakage sensor for the detection of oil that comprises a strip-shaped sensor material embedded into a flat housing which in turn is embedded in an oil-absorbing material. The strip-shaped sensor material is connected to an electrical supply line at both ends. The sensor material may consist of a mixture of an electrically conductive substance, such as carbon, with porous polytetrafluorethylene (PTFE). If wetted with oil, the sensor strip will change its electrical resistance, and this effect is utilized for leakage detection. The flat housing into which the strip-shaped sensor material is embedded consists of porous PTFE so that the leaking oil penetrates up to the strip-shaped sensor material. The oil-absorbing material into which the sensor housing is embedded may be a textile material, such as isotactic polypropylene. The material absorbs oil almost instantly, but hardly absorbs any water. This oil-absorbing material ensures that leaking oil reaches the strip-shaped sensor material over large surfaces and that the oil is held long enough to reach the sensor material. As a consequence, the sensitivity of the leakage sensor is increased.
The oil-absorbing material into which the sensor itself is embedded is accommodated in a protective tube which has openings to allow the passage of leaking oil and an outlet for the electrical lines connected with the sensor strips.
In the conventional leakage detection system shown in FIG. 1, each pair of the leakage sensors 11 is connected to an individual evaluation unit 15 through two supply cables 13 of individual length. The evaluation units 15 determine and process changes of the resistance of the leakage sensors 11. Since the evaluation unit 15 and the leakage sensor 11 may be located at a considerable distance from each other, there is the problem that resistance changes must be measured through long supply cables. Due to the intrinsic resistance of the supply cable, the maximum possible distance between the leakage sensor 11 and the evaluation unit 15 is limited. External influences on the long supply line, such as electromagnetic interference and voltage changes disturb the measuring process, which may lead to a false alarm. If such a leakage detection system is used in an explosion-prone area, the supply line must be limited in length due to the maximum admissible capacity in this area. If the supply line is too long and the capacity too high, there is a danger of capacitative charge and spark formation when the capacity is discharged. Supply lines suitable for measuring are expensive. The relatively high costs of measuring lines are often reflected in the total costs of the leakage detection system because every leakage sensor 11 and the corresponding evaluation unit 15 must be connected through an individual supply cable 13.
The high investment costs for such a leakage detection system are aggravated by the relatively high installation and service costs. A separate supply cable 13 must be laid for each leakage sensor 11. High installation costs result due to the fact that individual adjustment is required for each leakage sensor to take into account the individual length of each corresponding supply cable 13. This is not only very time-consuming but also requires well trained and cost-intensive personnel. Furthermore, costs accure because for each false alarm, a service engineer must go to the place of the suspected leakage and check it.
There is a need for a system to combine several leakage sensors in groups with a serial data bus.